Actuators of this kind are known through for instance EP A1 0 040 976 (Warnett) and U.S. Pat. No. 4,087,074 (Massey et al).
An actuator of this kind forms a compact and reliable unit for controlling various industrial processes and different kinds of apparatus.
Such an actuator is normally used to regulate valves, e.g. ball valves. In its simplest form, the actuator can, for example, rotate the actuating member through 90.degree., said member being connected to the valve slide, either directly or indirectly.
The actuating member can be rotated against the action of a spring, whereupon, if the fluid pressure is relieved, the pistons are returned to their zero-position by means of springs arranged in the rack portions.
The pistons, however, can also be returned to their zero-position under the influence of fluid pressure, either in addition to the springs or in place of said springs, whereupon the cylinder space between the end-walls of the actuator and respective pistons is placed under pressure via passages arranged in the cylinder wall and the end-walls, while de-pressurizing the space between the pistons.
It is often found subsequent to mounting the actuator, that the terminal positions of the pistons need adjusting, particularly when the actuator is to be used for regulating valves.
Hitherto, it has only been possible to adjust the outer terminal positions of the pistons, i.e. the positions in which the space between the pistons is under pressure and the pistons are located in the vicinity of their respective adjacent end-walls.
In this respect, it is possible to make such adjustments with known actuators by arranging in one of the end-walls of the actuator, preferably centrally of said wall, an adjusting means in the form of a screw, which projects away from said end-wall, into the cylinder chamber.
Thus, if the surface of the piston is located at a distance from the inner surface of the end-wall when said piston strikes against the screw, it is possible to adjust the stroke of the piston by turning the screw once or twice in one or the other direction. In practice, this normally corresponds to a 2.degree.-3.degree. rotation of the actuating member.
At present there is no possibility of making a similar adjustment when the pistons are located in their fully retracted position, corresponding to the zero-position of the actuating member.